to determine the extent of the source layer described Qi = instantaneous vorticity component in the Hunt-Graham model. The energy spectra show qualitative agreement with the model, though higher Subscripts resolution calculations will be required to make more i = 1,2, 3, coordinate directions quantitative comparisons. Additionally, the proxims = value at free surface ity of the free surface to the bottom solid wall of the oo = value in free stream channel evidences itself as a wall-layer streaky strucw = value at wall ture which persists to a noticeably greater distance. away from the wall. Some speculations are offered to 1. INTRODUCTION explain this effect. ,The study of the structure of turbulence near afree surface is obviously important to our understanding of the complex interaction of the atmosphere and upper ocean. It is also of fundamental relevance to the h = channel height -K wall-bounded turbulence problem, since it isolates the k = turbulent kinetic energy' -boundary influence on turbulent fluctuations from the e" = V/u 7 , viscous length scale turbulence production mechanism at the wall. The Reh = -h/v, Reynolds numbr first detailed experiment which addressed itself to this R' = uh/v, wall Reynolds number particular problem was that of Uzkan and Reynolds' Re = U,0/v, momentum thickness (UR). They passed grid generated homogeneous turReynolds number bulence over a wall which moved with the mean flow = V/u2, viscous timescale and therefore generated no mean shear at the bound-U = instantaneous velocity vector ary. They found that the streamwise turbulence inUi = instantaneous velocity component tensity near the shear-free boundary did not peak as u, = fluctuating velocity component it does near a stationary solid wall, but instead deur = V/I, friction velocity creased monotonically from its free stream value to A,, = turbulent microscale zero at the boundary. The simulations performed here were designed to vorticity vector is defined by fl = (V x U). Followrepresent as closely as possible the physics of free suring the solution of equations 1 and 2, the streamwise face/turbulence interaction in which the effects ofsurand spanwise velocity components, Ul and U 3 . are face waves can be safely neglected. For this purpose, recovered from the incompressibility condition. fully developed turbulence between a solid wall and aThe equations of motion are solved in Fourierfree surface is simulated. The physical processes repChebyshev space where Fourier modes are employed resented by these simulations differ in some important in the horizontal plane and Chebyshev modes in the respects from processes involved in the physical experwall normal direction. The calculations are performed iments noted above. First, in these simulations, no on a 64 x 65 x 48 grid in X 1 ,X 2 ,z 3 respectively. viscous layer can develop since u 1 and U3, the fluctuWith the geometry scaled by the channel height, the ating streamwise and spanwise velocity components, streamwise, vertical, and transverse dimensions of the are not forced ...
